Prestellar Cores in Turbulent Clouds: Observational Perspectives on Structure, Kinematics, and Lifetime

Abstract

We analyze an ensemble of simulated prestellar cores to facilitate interpretation of structure, kinematics, and lifetime of observed cores. While our theory predicts a "characteristic" density for star formation, it also predicts that the individual critical density varies among cores; any observed sample thus contains cores at various evolutionary stages within a given density bin. By analyzing the remaining lifetime, we find cores undergoing quasi-equilibrium collapse evolve on a timescale of twice the freefall time throughout most of their life. Our analysis shows that the central column density and the associated full-width half maximum provide a reasonably accurate observational estimator of the central volume density, and therefore the freefall time; this does, however require resolving the central column density plateau. Observations with a finite beam size tend to underestimate densities of evolved cores, and this makes observed lifetimes appear to decrease more steeply than the apparent freefall time. We measure from our simulations the ratio of prestellar duration to envelope infall time, and find this is consistent with the observed relative number of prestellar cores and embedded protostars. Yet, the absolute core lifetime in our simulations is significantly shorter than would be expected from empirical measurements of the relative numbers of prestellar cores and Class II sources; we discuss several possible reasons for this discrepancy. Finally, our simulated cores have nearly constant line-of-sight velocity dispersion within the emitting region in the sky plane, resembling observed "coherent cores." We show that this "coherence" is a consequence of projection effects, which mask the intrinsic power-law velocity structure function. We discuss possible ways to estimate line-of-sight path lengths.